Re: A really simple and readable C++ solution，only cost 12ms

Thanks for sharing! I learn this so-called Flood Fill algorithm elsewhere. Very smart and efficient since it avoids many unnecessary attempts in DFS if we don't pre-process 0 at the boundary first. Here is my Java version for your reference.

```
public void solve(char[][] board) {
if (board.length == 0 || board[0].length == 0) return;
int m = board.length, n = board[0].length;
// 1.Flood fill all 'O' on the boundary
for (int i = 0; i < board.length; i++) {
floodFill(board, i, 0);
floodFill(board, i, board[0].length - 1);
}
for (int i = 0; i < board[0].length; i++) {
floodFill(board, 0, i);
floodFill(board, board.length - 1, i);
}
// 2.Flip all left 'O' to 'X' and recover flooded '#' to 'O'
for (int i = 0; i < board.length; i++)
for (int j = 0; j < board[0].length; j++)
board[i][j] = (board[i][j] == '#') ? 'O' : 'X';
}
private void floodFill(char[][] board, int i, int j) {
Stack<int[]> stack = new Stack<>();
stack.push(new int[]{ i, j });
while (!stack.isEmpty()) {
int[] pos = stack.pop();
i = pos[0];
j = pos[1];
if (board[i][j] != 'O') continue;
board[i][j] = '#';
if (i > 0) stack.push(new int[]{ i - 1, j });
if (j > 0) stack.push(new int[]{ i, j - 1 });
if (i < board.length - 1) stack.push(new int[]{ i + 1, j });
if (j < board[i].length - 1) stack.push(new int[]{ i, j + 1 });
}
}
```